This invention is directed to an improved process for the recovery of nitrile monomer from the reactor effluent from a hydrocarbon ammoxidation reactor. More particularly, the invention is directed to an improved process for the recovery of nitrile monomer contained in the effluent stream from the ammoxidation of propylene, propane, isobutane or isobutylene in the production of acrylonitrile or methacrylonitrile.
The processes widely used in commercial practice for recovering the products of hydrocarbon ammoxidation generally comprise the steps of: a) contacting the effluent from an ammoxidation reactor in a quench tower with an aqueous quench liquid to cool the gaseous effluent; b) contacting the quenched effluent with water in an absorber, forming an aqueous solution comprising the ammoxidation products; c) subjecting said aqueous solution to extractive distillation with water in the recovery column to separate the crude monomer as an overhead vapor stream and collect water soluble, less volatile contaminants in a liquid waste stream from the bottom of the column; and d) condensing an overhead vapor stream to form an organic phase comprising nitrile monomer and an aqueous phase, and decanting the organic phase containing crude monomer. Further purification of the nitrile monomer may be accomplished by passing said organic phase to a second distillation column to remove at least some impurities from the crude nitrile monomer, and further distilling the partially purified nitrile in a third distillation column to obtain the purified acrylonitrile or methacrylonitrile.
Processes for recovery and purification of acrylonitrile and methacrylonitrile are well known and widely described in the art, for example, in U.S. Pat. Nos. 6,107,509, 4,234,510, 3,885,928, 3,459,639, 3,352,764, and 3,198,750. The entire disclosure of each of said patents is incorporated herein by reference.
Hydrocarbon ammoxidation, particularly of alkanes, is typically conducted using substantial excesses of ammonia. Ammonia that is not consumed in the ammoxidation exits the reactor in the effluent, together with nitrile monomer and reaction by-products including hydrocyanic acid, cyanoalkane and the corresponding aldehyde and the like. The by-products react with nitrile monomer in the presence of unreacted ammonia, or with one another. It is therefore necessary to separate the ammonia from the effluent stream immediately after the stream exits the ammoxidation reactor. Conventionally, the unreacted ammonia is removed in the form of a salt as a part of the quench operation, step a, by including sufficient acid, for example, sulfuric acid, in the aqueous quench liquid to neutralize and capture the excess ammonia. Depending upon several process factors including the ability to accurately meter and control the addition of the several reaction components and the reaction parameters, substantially all of the excess ammonia will be captured in the quench step.
The aqueous solution obtained from the absorber in the subsequent absorption step (step b) will comprise the water soluble components of the effluent including nitrile monomer, the corresponding coproducts including alkylnitrile and hydrocyanic acid, together with minor amounts of contaminants. For example, in an acrylonitrile process, the solution will include acrylonitrile, acetonitrile and HCN, together with a minor amounts of acrolein and other carbonyl compounds, residual ammonia, cyanohydrins and other contaminants. The pH will vary, depending in part upon the relative levels of HCN and residual ammonia.
Acrylonitrile and valuable coproducts including acetonitrile are separated in the recovery column. The separation is greatly affected by the pH of the system, which will preferably be maintained in the range of from about 5.5 to about 7.5, more preferably from about 6 to about 7. Within the preferred pH range, acrolein and other carbonyl contaminants react with water and HCN, forming high molecular weight, water soluble products that remain with the aqueous phase. Maintaining the system pH in the preferred range will also neutralize other volatile contaminants including nitrogen oxides, removing them from the product stream. A more alkaline condition promotes formation of HCN trimers and polymers, and leads to loss of product through nitrile hydrolysis and polymerization of nitrile monomer. Side reactions form high molecular weight products that may be carried into the column and lead to fouling of the column. At a low pH, acrolein and other volatile contaminants may escape to the overhead and contaminate the product stream.
The recovery column may be maintained at a near neutral pH by monitoring the pH and adding an appropriate amount of an alkaline compound to the system as required. The addition may be made at any convenient point; typically, the addition will be to the aqueous phase in the overhead decanter and to the solvent water fed to the column.
A variety of water soluble caustic additives including alkali metal hydroxides such as potassium hydroxide and sodium hydroxide have been disclosed in the art for this use, as have ammonia, alkyl amines and the like. However, these additives tend to react with one or more of the products. Contacting the product stream with strong caustics may initiate polymerizations and other side reactions; to avoid these difficulties, the caustic will be well-diluted with water, further adding to the disposal problem. Ammonia and water soluble alkyl amines undergo cyanoethylation by the nitrile monomer, and amines may be sufficiently soluble in acrylonitrile to be carried into the product stream.
Salts of weak acids, for example, alkali metal and alkaline earth metal carbonates, bicarbonates, acetates, phosphates and the like have also been disclosed for this use, for example, in U.S. Pat. No. 3,896,740. These mildly alkaline compounds may be added directly to the column or to a process stream without causing significant product loss. The additive most commonly employed in commercial processes for this purpose is soda ash, in part because of its low cost and ready availability. The residual salts resulting from the neutralization accumulate in the still bottoms and are eventually purged from the process as wastewater. Where the neutralizing additive is soda ash or caustic soda, the wastewater will necessarily contain a high level of sodium salts.
Acrylonitrile plants generate a significant volume of wastewater containing organic compounds, ammonia, and inorganic salts. Methods for treating or disposing of these streams include thermal or catalytic incineration, biotreatment, wet oxidation and reduction, and deep welling. These disposal methods represent current general industry practice. Producers of acrylonitrile are, however, studying alternative methods of wastewater handling. Currently, much of the wastewater generated by acrylonitrile plants in the United States is disposed of by the deep welling of streams with low levels of contaminants and the incineration of streams with higher levels of impurities.
Incineration of aqueous wastes containing high levels of sodium gives rise to serious corrosion problems, including erosion of the incinerator furnace refractory liner. Periodic shut-downs of the plant are needed to allow repair of the incinerator and replacement of the refractory, substantially adding to operating costs. Substantially reducing the level of sodium in effluent wastewater, thereby reducing damage to the incinerator and increasing the time between shutdowns, would thus provide a significant economic benefit.